Study on fracture behavior of layered limestone under mixed mode I/III loading

Fractures in anisotropic rocks pose challenges in rock engineering. However, fracture characteristics of layered rocks under mixed mode I/III loading are scarcely investigated. In this study, pure mode I, III, and mixed mode I/ III fracture tests were conducted using edge notch disc bend (ENDB) specimens made of layered limestone with different bedding angles (alpha = 0(degrees), 30(degrees), 45(degrees), 60(degrees), and 90(degrees)). The results showed that the equivalent fracture toughness (Keff) followed the trend: Mode I > Mixed Mode I/III > Mode III at alpha < 45, while the opposite trend is observed at alpha >= 45. Keff decreased and then increased with alpha under mode I and mixed mode I/III loading, increased first then decreased and finally increased with alpha under mode III loading. Conversely, the fracture energy trended: mode III > mixed mode I/III > mode I. The acoustic emission (AE) characteristics of the specimens were significantly influenced by the bedding angle alpha and loading mode as well as bedding strength. The AE signal for alpha = 0 was most conspicuous among the loading modes and followed by tensile/torsional damage and fracture along the bedding plane, and least when fracture only occurred along the bedding plane. The fracture trajectory depended on the combination of K-Ic, K-IIIc, and bedding strength. Generally, in pure mode I loading, the failure mode of the specimen changed from complete layer penetration to along the bedding plane as alpha increased. In contrast, in pure mode III and mixed mode I/III loading, the step-like failure pattern of anti symmetric twist transformed along the bedding plane as alpha increased. Furthermore, the MTS criterion agrees better with the K-III/K-Ic test values for alpha = 30(degrees) and 90(degrees), and the MTSED criterion agrees better with the test results for alpha <= 30(degrees). Test results could provide new insights into the mixed mode I/III fracture mechanism of layered rocks.